Thermoplastic fibers (most significantly, polypropylene fibers) are utilized in various end-uses, including carpet backings, scrim fabrics, and other fabrics for article reinforcement or dimensional stability purposes. Other thermoplastics, such as polyesters, polyamides, and the like, are mostly used in apparel fabrics, draperies, napery fabrics, and the like, as well. Unfortunately, prior applications utilizing standard thermoplastic fibers have suffered from relatively high shrinkage rates, due primarily to the fiber constituents. Heat, moisture, and other environmental factors all contribute to shrinkage possibilities of the fibers (and yarns made therefrom), thereby causing a residual effect of shrinkage within the article itself. Thus, although such polypropylene fibers are highly desired in such end-uses as carpet backings, unfortunately, shrinkage causes highly undesirable warping or rippling of the final carpet product. Or, alternatively, the production methods of forming carpets (such as, for example, carpet tiles) compensate for expected high shrinkage, thereby resulting in generation of waste materials, or, at least, the loss of relatively expensive amounts of finished carpet material due to expected shrinkage of the carpet itself, all the result of the shrinkage rates exhibited by the carpet backing fibers themselves. Furthermore, such previously manufactured and practiced fibers suffer from relatively low tensile strengths. For scrim fabrics (such as in roofing articles, asphalt reinforcements, and the like), such shrinkage rate problems are of great importance as well to impart the best overall reinforcement capabilities to the target article and permitting the reinforced article to remain flat. Utilization of much more expensive polyesters and polyamides as constituent fibers has constituted the only alternative methods to such problematic high shrinkage fibers in the past (for both carpet backings and scrim applications). Such replacement fibers, however, are not only more expensive than polypropylene fibers, but their tensile modulus levels are sometimes too low for certain desired end-use applications.
There has been a continued desire to utilize such polypropylene fibers in various different products (as alluded to above), ranging from apparel to carpet backings (as well as carpet pile fabrics) to reinforcement fabrics, and so on. Such polypropylene fibers exhibit a certain high level of high strength characteristics and do not easily degrade or erode when exposed to certain “destructive” chemicals. However, even with such impressive and beneficial properties and an abundance of polypropylene, which is relatively inexpensive to manufacture and readily available as a petroleum refinery byproduct, such fibers are not widely utilized in products that are exposed to relatively high temperatures during use, cleaning, and the like. This is due primarily to the aforementioned high and generally non-uniform heat- and moisture-shrink characteristics exhibited by typical polypropylene fibers. Such fibers are not heat stable and when exposed to standard temperatures (such as 150° C. and 130° C. temperatures), the shrinkage range from about 2% (in boiling water) to about 3-4% (for hot air exposure) to 5-6% (for higher temperature hot air). In addition, when polypropylene tapes and monofilaments are processed in order to give relatively high tenacity and tensile modulus, the shrinkage can be even more dramatically higher, up to 20% at 150° C. These extremely high and varied shrink rates thus render the utilization and processability of highly desirable polypropylene fibers very low, particularly for end-uses that require heat stability (such as carpet pile, carpet backings, molded pieces, and the like). Furthermore, in high strength (high tenacity, high modulus, etc.) applications, such polypropylene fibers generally lack the requisite high strength physical characteristics needed to withstand external forces to permit utilization within a cost-effective article.
Past uses of polypropylene fibers within carpet backings have resulted in the necessity of estimating nonuniform shrinkage rates for final products and thus to basically expect the loss of a certain amount of product during such manufacturing and/or further treatment. For example, after a tufted fiber component is first attached to its primary carpet backing component for dimensional stability during printing, if such a step is desired to impart patterns of color or overall uniform colors to the target tufted substrate. After printing, a drying step is required to set the colors in place and reduce potential bleeding therefrom. The temperatures required for such a printing step (e.g., 130° C. and above) are generated within a heated area, generally, attached to the printing assembly. At such high temperatures, typical polypropylene tape fiber-containing backings exhibit the aforementioned high shrink rates (e.g., between 2-4% on average). Such shrinkage unfortunately dominates the dimensional configuration of the printed tufted substrate as well and thus dictates the ultimate dimensions of the overall product prior to attachment of a secondary backing. Such a secondary backing is thus typically cut to a size in relation to the expected size of the tufted component/primary backing article. Nonuniformity in shrinkage, as well as the need to provide differently sized secondary backings to the primary and tufted components thus evince the need for low-shrink polypropylene tape fiber primary carpet backings. With essentially zero shrinkage capability, the reliable selection of a uniform, proper size for the secondary backing would be a clear aid in reducing waste and cost in the manufacture of such carpets.
If printing is not desired, there still exist potential problems in relation to high-shrink tape fiber primary backing fabrics, namely the instance whereupon a latex adhesive is required to attach the remaining secondary backing components (as well as other components) to the tufted substrate/primary backing article. Drying is still a requirement to effectuate quick setting of such an adhesive. Upon exposure to sufficiently high temperatures, the sandwiched polypropylene tape fiber-containing primary backing will undergo a certain level of shrinkage, thereby potentially causing buckling of the ultimate product (or other problems associated with differing sizes of component parts within such a carpet article). And, again, tensile strength, tenacity, and modulus are generally unavailable at sufficiently high levels with simultaneous low-shrink properties. Thus, past low-shrink fibers have been highly suspect as proper selections for high-strength end-use fabrics.
To date, there has been no simple solution to such problems, even a fiber that provides merely the same tensile strength exhibited by such higher-shrink fibers. Some ideas for improving upon the shrink rate characteristics of polypropylene fibers have included narrowing and controlling the molecular weight distribution of the polypropylene components themselves in each fiber or mechanically working the target fibers prior to and during heat-setting. Unfortunately, molecular weight control is extremely difficult to accomplish initially, and has only provided the above-listed shrink rates (which are still too high for widespread utilization within the fabric industry). Furthermore, the utilization of very high heat-setting temperatures during mechanical treatment has, in most instances, resulted in the loss of good hand and feel to the subject fibers, and also tends to reduce the stiffness. Another solution to this problem is preshrinking the fibers, which involves winding the fiber on a crushable paper package, allowing the fiber to sit in the oven and shrink for long times, (crushing the paper package), and then rewinding on a package acceptable for further processing. This process, while yielding an acceptable yarn, is expensive, making the resulting fiber uncompetitive as compared to polyester and nylon fibers. As a result, there has not been any teaching or disclosure within the pertinent prior art providing any heat- and/or moisture-shrink improvements in polypropylene fiber technology.
As noted above, the main concern with this invention is the production of low-shrink, high-tenacity, high tensile strength, high modulus strength thermoplastic fibers. For the purpose of this invention, the term “thermoplastic fiber” or fibers is intended to encompass polyester, polyamide, or polyolefin monofilament fibers. As noted above, such a fiber is generally produced through the initial creation of a thermoplastic resin (such as a polypropylene, a polyolefin) from which the desired fibers are extruded into individual fibers that can then be incorporated into yarns, fabrics, or both. To date, no thermoplastic fibers exhibiting simultaneous low-shrink and high-modulus strength (high-tenacity) characteristics have been accorded the pertinent markets.
Additionally, it has been noted that prior polypropylene fibers, particularly tape and/or monofilament types, fail to provide sufficient degrees of creep-strain for certain end use availability. Fibers that do permit utilization within applications such as geotextiles, and the like, require reliable creep-strain characteristics for proper functioning. Polypropylene fibers performing at a level high enough for long-term reliability have not been forthcoming within this industry. Furthermore, fibers that exhibit proper crystal configurations (such as measured and analyzed by small angle X-ray scattering) to provide improved resistance to stretching, deforming, and other potentially destructive physical result during standard use have not been disclosed within the prior art.